Instant startup, even with thousands of tracks
Easy to use directory browser
Customizable colors
Dynamic keybindings. You can bind a key to any command, :seek +1m for example
Vi / less style search mode
Vi style command mode with tab completion

Misc

Excellent compilations handling
Uses Unicode internally for all string handling
Can run external commands for the currently selected files (tag-editor for example)
Can be controlled via UNIX socket using cmus-remote command
Known to work on Linux, OS X, FreeBSD, NetBSD, OpenBSD and Cygwin